1. Field of the Invention
The instant invention generally relates to a fluid control mechanism by which the frequency constituents within an acoustic signal are converted to a useful working output. More specifically, the instant invention relates to a fluid control mechanism including a resonance chamber that produces oscillatory flow of a working fluid in response to exposure to an acoustic signal.
2. Description of the Related Art
A wide variety of actuating technologies have been developed for use in miniaturized systems for the life sciences including integrated microfluidic system. For example, the integrated microfluidic systems may be used to produce microgradients of liquid reagents and samples. The microgradients of the liquid reagents and samples may be utilized for understanding many of nature's developmental processes.
Control and transport of the liquid reagents and samples are difficulties that are often encountered with the integrated microfluidic systems. Most known integrated microfluidic systems rely heavily upon external liquid or air pressure to transport the liquid reagents and samples between dedicated fluidic unit operations in the systems. Use of the external liquid or air pressure often requires the use of extensive external control equipment, and difficulties with control of fluid flow often arise due to the use of multiple pumps dedicated to each fluidic unit.
Manipulation or control of discrete fluid droplets has been performed using air pressure with careful attention paid to a magnitude of the pressure gradient as most pressure regulators are not configured or designed to output minute pressure differences that are needed for precise in vivo droplet control. A related approach to droplet control employs intermittent pulsing of a coarsely regulated pressure source to precisely position droplets. Another approach that has been taken with regard to distributed pressure control utilizes micro-machined Venturi pressure regulators. Hybrid schemes employing both displacement and direct pressure are also possible, most notably, for use in serial deflection of elastomeric membranes. With this approach, multiplexed pressure control is feasible, but the number of external connections and control equipment required to operate a reasonably complex integrated microfluidic system is prohibitively large in size, and such an approach also requires high power actuation schemes.
The dependence on external liquid or air pressure is becoming increasingly problematic with the push towards integrated microfluidic systems, which can include thousands of independent pressure regulators. Additionally, the lack of low power actuation schemes has, in part, hindered the use of the systems for various applications.
Fluid control schemes that utilize acoustics are known in areas ranging from fluid transport, mixing, separations, and droplet levitation. Two relevant fluid control schemes utilizing acoustics are acoustic streaming and surface acoustic waves (SAW). Acoustic streaming, also known as quartz wind, is a phenomenon by which a steady momentum flux is imparted to a fluid due to the impingement of high amplitude acoustic waves. Bulk motion of the fluid results from a build up of a non-linear viscous Reynolds stress. However, due to an intolerance to back pressure, microfluidic applications using acoustic streaming have thus far been limited primarily to driving closed-loop fluid circuits. SAWs, on the other hand, operate principally on an open planar surface rather than within a closed channel. Surface confined acoustic waves can be launched within piezoelectric substrates by applying resonant frequencies to sets of interdigitated electrodes with the resonance frequencies determined by electrode spacing. SAWs are launched perpendicular to the electrodes and decay rapidly with substrate depth but decay negligibly in the direction of propagation. Surface bound droplets in the path of a SAW undergo a rolling motion due to acoustic streaming that occurs at a leading pinned meniscus of the droplet. As such, SAWs can be used to position droplets arbitrarily along lines of intersecting electrode paths.
One limitation to the use of SAWs, in addition to potential limitations introduced from use of an open platform (such as reagent and sample storage, evaporation losses, contamination), is fabricating the numbers of electrodes necessary for precise droplet placement.
Another type of fluid control scheme that utilizes acoustics is an acoustic compressor. In acoustic compressors, the exposure of a resonance chamber to an acoustic signal containing a tone at a frequency that is substantially similar to the resonance frequency of the resonance chamber creates pressure oscillations within a gas-filled cavity of the resonance chamber. These pressure oscillations have been typically converted into compression and flow by reed valves that are attached to the resonance chamber. The gas oscillates back and forth in the cavity, alternately compressing and rarifying the gas. The displacement of this gas can be changed by varying the power input, thus resulting in variable pumping capacity. However, the acoustic compressors require an inlet and an outlet to the resonance chamber to avoid buildup of pressure in the resonance chamber. Further, the acoustic compressors generally require a large size of the cavity to keep the operating frequencies within the range of practical reed valves. As such, acoustic compressors tend to be physically large for a given pumping capacity, when compared to other types of compressors, which is especially detrimental for microfluidic systems.
Due to the deficiencies of known schemes used to control fluid flow in integrated microfluidic systems, there is an opportunity to develop new schemes that overcome such deficiencies.